1. Field of the Invention
This invention relates to magneto-optic probes and, more particularly, to measuring the magneto-optic polarization rotation in a thin film magnetic film for imaging the spatial and temporal current distribution in an adjacent material such as an integrated circuit.
2. Background Art
Presently, integrated circuits are tested by making electrical connection to contacts or pads provided on an integrated circuit chip and applying electrical signals to selected pads and monitoring the output signals at other selected pads. In the event of an electrical failure on the integrated circuit or where the integrated circuit does not meet normal specifications, it is difficult to locate within the integrated circuit the actual currents and waveforms which result in the output signal. Further, some integrated circuits have very high speeds and the capacitance of the pads nullify the high speed waveforms present at internal nodes in the integrated circuit. New, non-destructive methods for monitoring the currents within an integrated circuit chip are, therefore, desirable.
A magnetic field passing through a material causes changes in the index of refraction for different polarization vectors, leading to polarization rotations (Faraday, Kerr effects) or intensity changes (magnetoabsorption). In magneto-optic materials these effects are large enough to be useful. A polarized light beam passing through a magneto-optic material will be subject to the change of the index of refraction resulting in an apparent rotation of the polarized light. The amount of rotation is an indication of the change of the index of refraction, which in turn is an indication of the instantaneous amplitude of the magnetic field.
U.S. Pat. No. 4,956,607 which issued on Sep. 11, 1990 to M. Abe et al. shows an arrangement including a polarizer, a magneto-optic element, an analyzer and a light sensitive element for measuring the polarization rotation of the light for measuring the current in a power line by directing a beam of light through the polarizer and magneto-optic clement in the direction of the magnetic field. The magneto-optical element may include yttrium-iron-garnet (YIG) material or ferromagnetic glass and lead glass.
U.S. Pat. No. 4,947,107 which issued on Aug. 7, 1990 to R. W. Doertier et al. describes a sensor for sensing the magnitude of current flowing in a conductor based upon the Faraday effect. A polarized light beam passes through a magneto-optic element without substantial internal reflection in the magneto-optic clement wherein the element rotates the plane of polarization of the input beam in proportion to a magnetic field coupled in parallel to the light beam.
U.S. Pat. No. 4,933,629 which issued on Jun. 12, 1990 to Y. Kozuka et al. describes a method and apparatus for measuring the strength of an AC electric field based on a light beam which is transmitted through and thus modulated by an optical sensing head due to the Pockel's and Faraday effects while the sensing head is exposed to the AC electric and magnetic fields. A polarized light beam passes through an optical element and then an analyzer and light sensitive element.
U.S. Pat. No. 4,823,083 which issued on Apr. 18, 1989 to P. L. Meunier describes a head for measuring of magnetic fields using the effect of the magnetic field on polarization of light. A light beam is directed through a ferromagnetic material such as yttrium-iron-garnet (YIG) wherein the light beam experiences a rotation of its planar polarization owing to the Faraday effect caused by the magnetic Field.
A class of materials which exhibit greatly enhanced magneto-optical light modulation is described in U.S. Pat. No. 3,418,036 which issued on Dec. 24, 1968 to F. Holtzberg et al. In U.S. Pat. No. '036, europium chalcogenides in pure form or in solid solutions with other rare earth chalcogenides have Far larger Verdet constants than other materials commonly known in the art. Europium chalcogenides may include, for example, europium sulfide, europium selenide, europium oxide and europium telluride.